This invention relates to programmable logic device integrated circuits, and more particularly, to computer aided design (CAD) tools for designing logic circuits in programmable logic devices.
Programmable logic devices are a type of integrated circuit that can be programmed by a user to implement a desired custom logic function. In a typical scenario, a logic designer uses computer-aided design (CAD) tools to design a custom logic circuit. These tools use information on the hardware capabilities of a programmable logic device to help the designer implement the custom logic circuit in the programmable logic device. When the design process is complete, the CAD tools generate configuration data. The configuration data is loaded into programmable logic devices to configure them to perform the desired custom logic function.
Conventional logic design tools provide logic designers with information on how well a given implementation of a custom logic design is expected to perform. Experienced designers can make adjustments to the design to improve performance. For example, if a designer is informed that a particular implementation of a custom circuit is consuming a large amount of power, the designer may use the logic design tools to make changes to the design that reduce power consumption.
Conventional programmable logic devices use multiple power supply levels. Most circuitry in a programmable logic device integrated circuit is located in the center or core of the device. This circuitry, which is generally referred to as core logic, is powered by a core logic power supply voltage commonly called Vccint. Commonly used values of Vccint are in the vicinity of 1.2 volts. Input-output circuitry surrounds the core logic and is powered using an input-output power supply voltage commonly called Vccio. In programmable logic devices in which the input-output circuitry supports multiple signaling standards, the value of Vccio is generally adjustable. A programmable logic device of this type can operate satisfactorily with a Vccio value of 3.3 volts or a Vccio value of 1.5 volts (as an example).
In some programmable logic device architectures, predriver circuitry is used to supply core logic signals to the input-output circuitry. The predriver circuitry strengthens core logic signals before these signals are driven off chip using the output buffer resources in the input-output circuitry. The power supply voltage used to power the predriver circuitry is sometimes referred to as Vccpd.
In conventional programmable logic devices such as programmable logic devices in the Stratix® II programmable logic device family sold by Altera Corporation of San Jose, Calif., the value of Vccpd is fixed at the maximum permissible Vccio value. While Vccio is permitted to range from 1.2 volts to 3.3 volts to accommodate different signaling standards, a fixed Vccpd value of 3.3 volts must be used. If other values of Vccpd are used, the programmable logic device may not function properly.
Other conventional programmable logic devices such as programmable logic devices in the Cyclone™ family sold by Altera Corporation, have internal power supply lines that permanently short together Vccio and Vccpd, so the value of Vccpd is clamped at Vccio. Different blocks of input-output circuitry on a Cyclone™ device may have different values of Vccio, but Vccpd and Vccio values cannot be varied independently.
The practice of fixing Vccpd at a particular value in conventional programmable logic devices tends to reduce device complexity and simplify the design process. However, substantial amounts of power are consumed by the predriver circuitry. For example, under certain power supply conditions, the predriver circuitry may consume significantly more power than the input-output circuitry.
It would therefore be desirable to provide logic design tools that help logic designers reduce predriver power consumption when designing custom circuits for programmable logic devices.